1. Field of the Invention
The invention relates to a voltage-limiting circuit. More particularly, it relates to a voltage-limiting circuit to maintain a voltage produced by a voltage generator.
2. Discussion of the Related Art
In electrically programmable memories, it is generally necessary to have a so-called programming voltage VB available in the integrated circuit, this voltage VB being higher than the normal supply voltage VCC of the circuit. For example, VCC is usually equal to 5 volts and VB to 7 volts at most.
In certain memories, the programming voltage VB is provided inside the integrated circuit, from the normal supply voltage VCC. For this purpose, a voltage multiplier circuit, also called a load pump, is used. This voltage multiplier circuit receives VCC and sets up a voltage VB higher than VCC.
The voltage multiplier circuits basically make use, quite simply, of diodes, two capacitors and a clock signal. In a first stage, the first capacitor is charged up to VCC and then it is discharged into the second capacitor. Then the same operation is started again and the voltage at the terminals of the second capacitor increases gradually.
The programming voltage will thus tend asymptotically towards a borderline or limit value, this limit value being equal to a multiple of the supply voltage VCC (twice VCC in the example described here above).
This type of assembly raises a problem if the nature of the voltage to be reached is the limit value. Indeed, the voltage produced increases ever less rapidly as and when the second capacitor gets charged. To limit the build-up time of the voltage produced to the desired value, it therefore becomes necessary to oversize the load pump. Thus, a limit value greater than the desire value is used. It is then necessary to place a voltage regulator at output of the pump. This regulator limits the voltage produced to the desired value. Furthermore, if the desired value is not a multiple of the value of the supply voltage, the presence of a regulator circuit of this kind becomes logically necessary.
There are two ways of limiting the programming voltage.
A first approach is to reduce without stopping the pump, the excess load given by the pump, once the desired value is reached, by connecting the output of the pump to a ground by means of one or more diodes for example,
This approach requires stopping the pump when the desired value is reached and starting it again when the voltage at the terminals of the second capacitor becomes excessively low (this entails the assumption that it is accepted that the voltage produced is within a certain range of values whereas, in the former case, this voltage will be constant).
A second approach has the advantage of consuming less power but makes it necessary, in practice, to produce two reference voltages and to use two comparison circuits to compare these reference voltages with the minimum and maximum voltages acceptable. To produce the reference voltages, Zener diodes are typically used. The second approach therefore entails penalties in terms of the amount of space occupied and in economic terms.